Generally, a paint coating system for use in coating work pieces like vehicle bodies is required to be able to reduce the amount of waste paint which has to be discarded at the time of a color change, and to be able to cope with a large number of paint colors. As a paint coating system which can meets these requirements, there has been known in the art a cartridge type paint coating system employing cartridges which are filled with different colors and adapted to be interchangeably attached to a coater unit (see, for example, Patent Literature 1: Japanese Patent Laid-Open No. 2000-176328, Patent Literature 2: Japanese Patent Laid-Open No. 2004-42036).
The cartridge type paint coating system, according to the prior art mentioned-above, is largely constituted by a paint coating machine including a coater unit for spraying paint which is supplied from loaded cartridge toward a coating object, and a paint replenishing system for a cartridge which is adapted to replenish an empty or consumed cartridge with paint which is supplied from a paint source by way of a paint supply line.
In this case, the paint replenishing system for a cartridge is provided with a paint feed means adapted to supply paint to a paint chamber which cartridge is defined in a paint chamber and an extruding liquid chamber by a movable partition wall, and an extruding liquid supply means adapted to push out residues of a previous color from the paint chamber by supplying an extruding liquid to the extruding liquid chamber of the cartridge.
As soon as a cartridge is set on a replenishing stool, an extruding liquid is supplied to an extruding liquid chamber in the cartridge from an extruding feed means to push out residues of a previous color from a paint chamber. In the next place, paint is replenished into the paint chamber of the cartridge by a paint feed means. At this time, the extruding liquid in the extruding liquid chamber is discharged and collected in an extruding liquid storage tank.
Generally, a paint replenishing system for a cartridge of this sort is provided with a flow meter in a paint conduit for the purpose of resilling a cartridge with a predetermined amount of paint which will be used in next coating operation. By means of this flow meter, the paint replenishing system for a cartridge can measure the flow rate of paint in supply to a cartridge, turning off the paint supply as soon as the cartridge is replenished with a suitable amount of paint which is determined depending upon the size of a work piece to be painted, coating film thickness and other conditions of a paint coating operation.
In this regard, according to the prior art mentioned above, a flow meter is provided in the course of a paint supply conduit thereby to measure the amount of replenishing paint. Taking into consideration the fact that there are a diversity of paint products which differ from each other in properties such as viscosity and specific gravity and which may or may not contain mica or a metallic component. Therefore, depending upon paint properties, errors can occur to measurements by a flow meter. Therefore, in the prior art, this problem is coped with by replenishing cartridges with paint in an amount which is larger than an actually necessary amount, precluding the possibilities of a shutdown of a coating operation due to paint shortage.
However, in case a cartridges is refilled with more than a necessary amount of paint, a larger amount of paint which still remains in the cartridge has to be discarded at the end of each coating operation, resulting in larger paint consumption and increases in running cost due to degradations in operational efficiency caused by necessities for heavier paint washing and disposing operations involving a larger amount of waste paint.
In this regard, it is conceivable to cut the amount of waste paint by returning a residual paint in a cartridge to a paint source. However, even so, it is technically difficult to return to a paint source all of residual paint in a cartridge including paint residues in a long paint conduit connected to the paint source. Paint residues in a paint conduit have to be discharged as a waste liquid at the time of a color changing operation.
On the other hand, in a case where each one of cartridges is used exclusively for a particular color, there is no necessity for washing a cartridge at the time of a color change. However, after repeated refilling operations, excess paint can remain in a cartridge as an old content which is degraded in quality to such a degree as to give adverse effects on finish quality and therefore may degrade the quality of coatings when mixed into fresh paint.